https://lmgtfy.com/?q=How+to+Set+Aut...tsApp+Business 
How to Set Auto Reply for WhatsApp Business
Step 1: Get AutoResponder for WhatsApp. Download & Install the AutoResponder for WhatsApp app from the Google Play Store on your Android device.
Step 2: Grant Notification Access. ...
Step 3: Create a New Rule. ...
Step 4: Set Auto Replies Based on Your Preference.